Conventionally, a fuel vapor treatment apparatus includes a canister accommodating an absorbent for temporarily absorbing fuel vapor produced in a fuel tank. The fuel vapor is removed from the canister as needed, so that the removed fuel vapor is purged into an internal combustion engine through an intake passage together with intake air. According to JP-A-H5-18326 and JP-A-H6-101534, an amount to fuel vapor purged to an intake passage is enhanced by detecting concentration of fuel vapor, which is to be purged into the intake passage, in advance. In such a fuel vapor treatment apparatus, mixture gas containing fuel vapor flows into the intake passage through a purge passage. The fuel vapor treatment apparatus controls the purge operation of fuel vapor by detecting a flow amount or a density of air in a passage, which opens to the atmosphere, in addition to detecting a flow amount or a density of the mixture gas in the purge passage.
In the fuel vapor treatment apparatus disclosed in JP-A-H5-18326 or JP-A-H6-101534, negative pressure in the intake passage is applied to each passage, so that the flow amount or the density is detected while flowing mixture gas or air into each passage. In each of these structures, when pulsation occurs in negative pressure in the intake passage, the flow amount and the density fluctuate. Consequently, the purge control cannot be accurately conducted on the basis of the detection of the flow amount or the density. When negative pressure in the intake passage is small, the mixture or air decreases in each passage. As a result, it is difficult to detect the flow amount or density.
It is conceivable to reduce pressure in a detection passage, which includes a throttle, using a gas flow generating unit to individually generate air flow and gas flow, so as to control the purge operation on the basis of detection pressure, which corresponds to the throttle and the gas flow generating unit. In such a fuel vapor treatment apparatus, detection pressure becomes stable, and the amount of air or gas through the detection passage can be sufficiently generated. Therefore, pressure can be properly detected, so that the purge operation can be accurately controlled. Thus, the air/fuel ratio in the engine can be protected against influence caused by the purge operation.
However, when the amount of fuel vapor absorbed in the canister exceeds an absorbing capacity of the canister, breakthrough occurs in the canister. When breakthrough occurs in the canister, fuel vapor is exhausted from the canister into an open passage, through which fuel vapor is exhausted to the atmosphere. For example, pressure of air passing through the throttle may be detected in a condition where the atmosphere passage communicates with the detection passage. In this operation, fuel vapor exhausted to the open passage may pass through the throttle, and consequently, the fuel vapor may flow into the gas flow generating unit. In this case, the characteristic of the gas flow generating unit may change. The gas flow generating unit may be a pump having an exhaust port communicating with the open passage. In this case, fuel vapor exhausted to the open passage may flow into the pump. Consequently, the P−Q characteristic of the pump may change. Thus, accuracy of pressure detection, or accuracy of controlling the purge operation may decrease.
A fuel vapor treatment apparatus may further include a second canister, in addition to a first canister for absorbing fuel vapor produced in the fuel tank. The second canister is provided in the detection passage between the throttle and the gas flow generating unit for absorbing fuel vapor.
In this fuel vapor treatment apparatus, the second canister absorbs fuel vapor flowing into the detection passage in a condition where pressure is detected. Therefore, fuel vapor can be restricted from flowing into the gas flow generating unit. Therefore, when a pump is provided as the gas flow generating unit, as shown in FIG. 2, the detection pressure ΔPgas, in a condition where only fuel vapor passes through the throttle, is equal to shutoff pressure Pt of the pump. The detection pressure ΔPair in a condition where only air passes through the throttle is equal to pressure in the intersection between the a ΔP−Q characteristic curve Cair of the throttle and the P−Q characteristic curve Cpmp of the pump. Therefore, as shown in FIG. 2, a detection gain G, which is a difference between the detection pressure ΔPgas, ΔPair, becomes large. Thus, accuracy of controlling the purge operation can be enhanced.
However, even in the structure, in which the second canister is additionally provided, breakthrough may occur in the second canister. When breakthrough occurs in the second canister, the second canister exhausts fuel vapor into the gas flow generating unit, as the gas flow generating unit generates gas flow. Consequently, fuel vapor may be drawn into the gas flow generating unit. As a result, the characteristic of the gas flow generating unit changes. Consequently, accuracy of detection pressure and accuracy of controlling the purge operation may decrease. Furthermore, the gas flow generating unit may be a pump having an exhaust port opening to the atmosphere. In this structure, the pump draws fuel vapor, and the fuel vapor is exhausted to the atmosphere. As a result, the fuel vapor causes air pollution.